Calcium-, phospholipid-dependent protein kinase activity of cultured rat Sertoli cells and its modifications by vitamin A

FSH regulates RA signaling to commit spermatogonia into differentiation pathway and meiosis

BACKGROUND

Spermatogenesis is a complex process that is controlled by interactions between germ cells and somatic cells. The commitment of undifferentiated spermatogonia to differentiating spermatogonia and normal spermatogenesis requires the action of gonadotropins. Additionally, numerous studies revealed the role of retinoic acid signaling in induction of germ cell differentiation and meiosis entry.

MAIN TEXT

Recent studies have shown that expression of several RA signaling molecules including Rdh10, Aldh1a2, Crabp1/2 are influenced by changes in gonadotropin levels. Components of signaling pathways that are regulated by FSH signaling such as GDNF, Sohlh1/2, c-Kit, DMRT, BMP4 and NRGs along with transcription factors that are important for proliferation and differentiation of spermatogonia are also affected by retinoic acid signaling.

CONCLUSION

According to all studies that demonstrate the interface between FSH and RA signaling, we suggest that RA may trigger spermatogonia differentiation and initiation of meiosis through regulation by FSH signaling in testis. Therefore, to the best of our knowledge, this is the first time that the correlation between FSH and RA signaling in spermatogenesis is highlighted.

Effects of follicle-stimulating hormone and vitamin A upon purinergic secretion by rat Sertoli cells

Follicle-stimulating hormone (FSH) and vitamin A (retinol) are two of the main regulators of the male reproductive system. Recently, it has been described that extracellular purines can affect some important reproductive-related functions in Sertoli cells and germinative cells, by activating specific purinergic receptors. In this work, we report that both FSH and retinol are able to induce changes in the levels of extracellular purines of cultured rat Sertoli cells. FSH induced an increase in adenosine, mainly caused by enhanced ecto-ATPase activity, while retinol increased xanthine and hypoxanthine levels, and decreased uric acid concentration by an unknown mechanism. These data indicate that purinergic signaling may be involved in the control and/or regulation of some of the reproductive-related actions of these hormones.

Phorbol myristate acetate induces changes on F-actin and vinculin content in immature rat Sertoli cells

Actin and vinculin are two of the most abundant cytoskeletal proteins, widely expressed in nearly all types of eukaryotic cells. It has been well established that long-term exposure to the tumor promoter phorbol myristate acetate (PMA) affects Sertoli cell morphology, as well as F-actin and vinculin organization in vitro. To analyze in a quantitative manner the F-actin and vinculin content of rat immature Sertoli cells in vitro in response to PMA exposure, cytoskeletal fractions were prepared following extraction with Triton X-100. Analysis of the isolated cytoskeletal fractions by immunoblotting showed that exposure of immature rat Sertoli cells to PMA for 8h has an appreciable effect on the cellular level of both the actin and vinculin. Interestingly, as revealed by using calphostin C, a specific protein kinase C inhibitor, the observed F-actin and vinculin changes are most probably mediated by a mechanism that depends on protein kinase activity. A discussion is made concerning PKC modulation by PMA and the subsequent actin and vinculin quantitative changes and reorganization, phenomena that have been closely related to cell transformation.

Protein kinase C regulation of glypican-1, syndecan-1 and syndecan-4 mRNAs expression during rat Sertoli cell development

Our previous studies indicated that cell surface proteoglycans were mostly heparan sulfate ones (HSPG) in 20 day-old Sertoli cells [Biochim. Biophys. Acta 1510 (2001) 474]. Among these HSPG, glypican-1, syndecans-1 and -4 mRNAs were expressed and differentially regulated. Glypican-1 and syndecan-1 mRNA expression was up-regulated under PKC activation in contrast to syndecan-4 mRNA expression which was not affected [Biochim. Biophys. Acta 1474 (2000) 31]. Rat Sertoli cells undergo extensive changes during the postnatal period both in structure and function, as the hematotesticular barrier establishment occurs at around 20 day-old. The testicular PKCalpha expression in developing Sertoli cells results in (i) a soluble (inactive) form which is maximal at the age of 1 day and declines gradually thereafter and (ii) a particulate (active) form which is low at birth, increases six-fold on days 8-11 of age and declines thereafter. The present study focused on the glypican-1, syndecan-1 and syndecan-4 mRNA expression and regulation under PKC activation by the phorbol myristate acetate (PMA) in 10-30 day-old Sertoli cells. Our data indicated that the regulation of their expression specifically depends on the nature of HSPG and Sertoli cell developmental stage and evidenced a specific PKC regulation of HSPG mRNA expression.

Protein kinase activity is central to rat germ cell apoptosis induced by methoxyacetic acid

Methoxyacetic acid (MAA) is a major metabolite of ethylene glycol monomethyl ether (EGME). Previous investigations of the testicular lesion induced by EGME have found that dividing meiotic cells are the most sensitive, although several stages of spermatocytes are also vulnerable. Preliminary data from this lab suggested the involvement of protein kinase activity in the development of this lesion, a hypothesis explored in the present studies. We used cultured seminiferous tubules (STs) from juvenile rats (25-day-old), exposed in vitro to MAA and several inhibitors of protein kinases. Nineteen h following a 5-h exposure to 5 mM MAA (the plasma level in vivo after a toxic dose of EGME), apoptotic spermatocytes were seen in early- and late-stage STs. Cell death was prevented by cotreatment with broad-spectrum inhibitors of protein kinases such as H-7, H-8, K-252a, W-7, and genistein. In corroboration, immunocytochemistry with antibodies to various kinases (PKCmu, zeta, and gamma, AKAP220, CaMKII, MLCK, and Src) showed increased staining around dying spermatocytes following EGME treatment in vivo. 2D-PAGE, autoradiography, and nanospray mass spectrometry was used to separate and identify proteins whose phosphorylation status was most greatly changed following exposure to MAA. One protein was identified by sequence analysis as being glucose-regulated protein 94 (grp94). Westem blotting and immunocytochemistry confirmed this finding. The data we present implicate kinase activities in the pathogenesis of this lesion and suggest the involvement of Sertoli cells.

Retinoid modulation of plasminogen activator production in rat Sertoli cells

Tissue type (t) and urokinase type (u) plasminogen activators (PAs) have been shown to be secreted by Sertoli cells in the seminiferous tubules in a cyclic fashion and to be dependent upon FSH stimulation or upon the presence of adjacent spermatogenic cells. In the present study we have analyzed the production of PAs by retinoid-treated rat Sertoli cells. In addition, because retinoids modulate the response of Sertoli cells to FSH either potentiating or antagonizing its action, we have investigated a possible modulation of FSH-stimulated PA production. Under basal conditions, Sertoli cells, isolated from prepubertal rats, secrete predominantly uPA. A significant dose-dependent inhibition of uPA activity was observed after treatment with retinol, while no significant effect was detected upon tPA secretion. When Sertoli cells were cultured in the presence of 0.25 microM retinol, a significant inhibition of uPA activity was evident after 16 h of treatment and reached approximately 80% after 48 h of treatment. The analysis of the mRNA levels revealed that retinol induces an inhibition of the steady-state levels of uPA mRNA without affecting those of tPA. Moreover, retinol affected uPA mRNA levels by increasing mRNA turnover. The effect of retinoids on Sertoli cells isolated from older animals was less evident, possibly due to the reduced production of uPA with the increase of age of the donor animals. Our results on the effect of retinoids upon Sertoli cell uPA production reinforce the importance of retinoids in the control of postnatal testis development.

Activation of protein kinase C increases proteoglycan synthesis in immature rat Sertoli cells

In order to determine the signal transduction pathways involved in the regulation of proteoglycan (PG) synthesis in immature rat Sertoli cells (SC), we have examined the effect of the tumor promoter phorbol ester PMA (phorbol myristate acetate) on [35S]sulfate and [3H]glucosamine incorporation into PG molecules neosynthesized by cultured rat SC. PMA induced a dose- and time-dependent stimulation of labeled cell-associated PG as determined by quantitative solid phase assay. The overall effect of PMA resulted from enhancement of both glycosylation and catabolism of cell PG, this latter effect leading to a drastic decrease of their residence time in the membrane. Besides these quantitative effects, activation of protein kinase C by PMA induced qualitative changes as reflected by increase in relative proportion of heparan sulfate PG (HSPG) in cell membrane PG. In light of our previous results suggesting an inverse relationship between PG synthesis and FSH responsiveness in immature rat Sertoli cells, the PMA-induced upregulation of cell membrane PG, and particularly HSPG, could constitute one mechanism involved in the repression of FSH-stimulated steroidogenesis induced by PKC activation.

The in vitro effect of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate on Sertoli cell morphology

The objective of the present study was to examine the effects of the well-known tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) on the morphology of cultured Sertoli cells from immature rats. The cells were cultured for 5 days and the TPA was added at the end of the culture period for 8 h at a concentration of 10-7 M. Viability tests showed that controls as well as TPA-treated cells remained viable during the culture period and no deleterious effects were observed as a result. Application of computerized morphometry at both light and electron microscopic levels revealed that TPA caused important changes in cell morphology in vitro. Statistical analysis of the results indicated that compared to the controls, Sertoli cells treated with TPA exhibited fewer astrocytic-type cytoplasmic extensions and a smaller size. Our results support the conclusion that the tumor promoter TPA, when applied to immature Sertoli cells in vitro, causes significant morphological alterations.

Calcium/phospholipid-dependent protein kinases in rat Sertoli cells: regulation of androgen receptor messenger ribonucleic acid

The possibility that Sertoli cell responses to testosterone are modulated by the calcium/phospholipid-dependent protein kinase (protein kinase C; PKC) was examined in rat Sertoli cells in culture. Both soluble and particulate cell fractions showed low constitutive phosphotransferase activity. Incubation with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA; 10(-7) M) was associated with a transient induction in both cell fractions of calcium/phosphatidylserine-dependent PKC activity, which was elevated from 15 min to 1 h. Consistent with this, mRNAs for the calcium/phospholipid-dependent isomeric forms of PKC (alpha, beta, and gamma) were detected. The expression levels of mRNAs for PKCalpha and PKCbeta were also up-regulated (2.5- to 3-fold) by TPA (10(-7) M), but these effects were much slower (peaking after 12 h) than those on phosphotransferase activity. In the presence of TPA (10(-7) M), expression of androgen receptor (AR) mRNA showed a transient time-dependent down-regulation ( approximately 70%), in which the nadir was reached after 6 h and baseline expression was again obtained after 12 h. The regulatory effect of PKC activation on AR mRNA was confirmed by the absence of response to a biologically inactive phorbol ester. A concentration-dependent decrease (half-maximal effect at approximately 10(-8) M TPA) of AR mRNA was also observed. These data suggest that Sertoli cell responses to testosterone may be inhibited by a transiently active PKC with a wide intracellular distribution.

Stable transfection of the rat follicle-stimulating hormone receptor complementary DNA into an immortalized murine Sertoli cell line

A plasmid expressing the rat FSH receptor (R) cDNA under the Simian virus (SV) 40 promoter/enhancer was stably transfected into a mouse Sertoli cell (SC) line (MSC-1) established from transgenic mice carrying a fusion gene of the human anti-Müllerian hormone (AMH) promoter sequences linked to the SV40 T-antigen gene (Peschon et al., 1992). The original cell line has numerous SC characteristics, but it was reported not to express the inhibin-alpha and follicle-stimulating hormone (FSH)R genes. The new FSHR expressing cell line possessed approximately 2000 per cell with equilibrium association constant (Ka) of 1.5 x 10(9) l/mol. In Northern blots, an FSHR mRNA species of 2.6 kb was found. The cells responded to recombinant human FSH (recFSH) and pertussis toxin (PT) with stimulated cAMP production. Moreover, PT enhanced the FSH-stimulated cAMP production in these cells, indicating the presence of a functional Gi protein. 12-O-tetradecanoylphorbol-13-acetate (TPA) suppressed the FSH-stimulated cAMP production of the cells, which effect was similar to that observed previously upon protein kinase C (PKC) activation in rat seminiferous tubules in vitro. Hence, the FSHR signalling, and its modulatory pathways, were intact in the FSHR expressing MSC-1 cell line. RT-PCR with inhibin-alpha specific oligonucleotide primers. followed by Southern hybridization, indicated that, unlike previously shown, the original and the FSHR expressing MSC-1 cells do express the inhibin alpha gene. FSH stimulation of the cells decreased their proliferation and, unexpectedly, the inhibin-alpha mRNA levels. The cells have functional features both from neonatal and mature SC. A feature of the former cells is the lack of FSH-stimulated up-regulation of inhibin-alpha expression; in fact FSH decreased this message. The antiproliferative, and apparently differentiating, effect of FSH on these cells resembled mature SC functions. Since adult SC do not proliferate in vitro, the new FSHR expressing and proliferating cell line provides a useful in vitro model for studying some facets of SC functions, though keeping in mind that these transformed cells do not behave identically with adult SC in vivo. The constitutive expression of FSHR in these cells allows the study of posttranscriptional events in the FSHR regulation.

Polyunsaturated fatty acids do not activate protein kinase C in the testis of the goldfish (Carassius auratus)

Earlier studies have established that polyunsaturated fatty acids (PUFA) such as eicosapentaenoic acid and docosahexaenoic acid inhibit steroid production in the goldfish testis. As PUFA inhibit testicular steroidogenesis in the rat through activation of protein kinase C (PKC), the present studies were undertaken to characterize the properties of PKC in the goldfish testis and to test the effects of selected PUFA on PKC activity. PKC activity was quantified in goldfish testis homogenate following partial purification by DEAE-cellulose chromatography by determining the transfer of radiolabelled phosphate from [γ - (32)P]ATP to histone III-S. Testicular PKC activity was defined by the amount of protein phosphorylation in the presence of phosphatidylserine, phasphatidylcholine, Ca(2+) ions and diolein (a 1,2-diacylglycerol analog) above that obtained in response to Ca(2+) ions alone. Western blot analysis of a crude testis homogenate using an antibody specific to the α and β isoforms of mammalian PKC led to the identification a single band of protein (80 kD) that co-migrated with PKC from rabbit brain cytosol. Addition of arachidonic, eicosapentaenoic or docosahexaenoic acids failed to activate PKC. However, PKC activity stimulated by phospholipid, Ca(2+) ions and diolein was inhibited in a dose related fashion by all of these fatty acids. These studies suggest that the inhibitory effects of EPA and DHA on testicular steroidogenesis are not mediated by activation of PKC. The lack of effect of PUFA on PKC activity in the goldfish testis suggests that either the distribution of PKC isoforms differs between the testis of mammals and fish or that PKC is not activated by PUFA in the goldfish.

Age-related variation of follicle-stimulating hormone-stimulated cAMP production, protein kinase C activity and their interactions in the rat testis

To study further the ontogeny of hormonal regulatory mechanisms in the testis, we measured follicle-stimulating hormone (FSH)- and protein kinase C (PKC)-stimulated cAMP production, PKC activity, and messenger (m)RNA levels of the PKC isoenzymes alpha, beta and gamma in rat testes between day 19 of fetal life and day 90 postpartum. Human FSH (30 mg/l) stimulated slightly but significantly cAMP production of fetal testes (57%; p < 0.05). A higher response (3-fold; p < 0.01) was observed on the day of birth, and the maximum FSH effect on cAMP (23-fold) was observed on day 10 postpartum. Thereafter, a gradual decline of FSH response occurred towards adult age. Concerning testicular PKC, the soluble (inactive) form had its maximum at the age of 1 day and this PKC form declined gradually thereafter. The particulate (active) form was low at birth, increased 6-fold on days 8-11 of age, and declined thereafter. A significant age-dependent variation was also found in the mRNA level of the PKC alpha isoenzyme (maximum on day 10), whereas those of PKC gamma and PKC beta were undetectable at all ages in Northern blots. When the in vitro modulation of basal and FSH-dependent cAMP production by the PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA, 100 nmol/l) was studied, the substance alone was without effect at all ages studied. The TPA effect on FSH-stimulated cAMP production displayed age-dependent variation: a slight stimulation in fetal testes, no effect at birth, decrease between days 8 and 11, and no effect on day 30.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of protein kinase C isoforms in various steroidogenic cell types

Different endocrine, steroidogenic cell types were examined for their content in protein kinase C (PKC) subtypes I, II and III. The expression of the PKC isoforms was assayed following high-performance liquid chromatography separation and characterization using a set of antipeptide antibodies specific for each enzyme isotype. Bovine and rat adrenocortical cells, as well as porcine Sertoli cells expressed only the type III PKC. By contrast, Leydig cell expressed both the isotypes I, II and II at similar levels. Taking into account the biological effect observed in these various cell types upon PKC activation, it may be suggested that the type III iso-PKC is involved in the steroidogenic activation pathways, whereas the expression of the types I, II, and III, only in Leydig cells, may contribute to a different array of cross-talk regulation pathways in these cells.

Inhibition of protein kinase C by oxidation products of retinol

Retinol has little effect on the activity of protein kinase C. However, air oxidation of retinol produces products which are inhibitory to this enzyme. These results are consistent with a suggestion that the activity of protein kinase C is modulated by the bulk biophysical properties of its environment. The facile susceptibility of retinol to oxidation, as demonstrated by HPLC analysis, explains some of the discrepant reports of its effects on the activity of protein kinase C.

Protein kinase C and Gi-protein mediated modulation of cAMP production in different stages of the rat seminiferous epithelium

The modulatory role of protein kinase C (PK-C)- and Gi-protein-mediated signal transduction systems was studied in the cyclic variation of follicle-stimulating hormone (FSH)-stimulated cAMP production of rat seminiferous tubules. FSH (Metrodin, Serono, 30 mg/l) stimulated cAMP production 10-fold (p less than 0.01) in a 3 h incubation of 5 mm segments of seminiferous tubules of stages II-VI of the epithelial cycle, but only 2-fold (p less than 0.01) in stages VII-VIII. The PK-C activator 12-O-tetradecanoylphorbol 13-acetate (TPA, 100 nmol/l) suppressed the FSH effect on cAMP output by 50-70% (p less than 0.01) in stages II-VI, but had no effect in stages VII-VIII. If the tubular segments were preincubated for 3 h in the presence of pertussis toxin (PT, 100 micrograms/l), the FSH-stimulated cAMP production of stages VII-VIII increased by 100-200% (p less than 0.01), and now they also became responsive to the TPA suppression. Conversely, no effect of PT was observed in stages II-VI. Cholera toxin (CT, 100 micrograms/l) and forskolin (Fk, 100 mumol/l) nearly similarly stimulated the cAMP production in both stages studied (about 10-fold, p less than 0.01), and TPA and PT potentiated the effects in a non-additive fashion. In conclusion, both Gi-protein and PK-C-mediated mechanisms modulate cAMP production of rat seminiferous tubules. A clear cyclic variation can only be demonstrated in FSH-stimulated cAMP production, but not if the Gs-protein or adenylate cyclase are directly stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)

Levels of binding proteins for retinoids in cultured Sertoli cells: effect of medium composition

The levels of cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP) have been measured in Sertoli cells maintained under different cultural conditions. Sertoli cells were isolated from prepubertal rats and cultured in a chemically defined medium without or with follicle-stimulating hormone (FSH), insulin, retinol or testosterone added individually or in combinations. The additions were made at the beginning of the culture or 24 h before the cells were subjected to determinations of CRBP and CRABP by radioimmunoassay. No differences were observed either after 1 or 4 days of treatment. The results obtained indicated that the levels of the two retinoid-binding proteins were unchanged in Sertoli cells in response to hormone and/or retinol administration. To rule out the possibility that the Sertoli cells used in our study were unresponsive to the hormones, lactate production by the cells cultured in the presence of FSH or insulin was measured. The amount of lactate produced under hormonal stimulation was significantly higher than the amount produced in absence of the hormones, thus indicating the ability of our Sertoli cells to respond to the hormonal stimulation.

Activity and regulation of calcium-, phospholipid-dependent protein kinase in differentiating chick myogenic cells

The activity of calcium-, phospholipid-dependent protein kinase (PKc) was measured in (a) total extracts, (b) crude membrane, and (c) cytosolic fractions of chick embryo myogenic cells differentiating in culture. Total PKc activity slowly declines during the course of terminal myogenesis in contrast to the activity of cAMP-dependent protein kinase, which was also measured in the same cells. Myogenic cells at day 1 of culture possess high particulate and low soluble PKc activity. A dramatic decline of particulate PKc activity occurs during myogenic cell differentiation and is accompanied, through day 4, by a striking rise of the soluble activity. The difference in the subcellular distribution of PKc between replicating myoblasts and myotubes is confirmed by phosphorylation studies conducted in intact cells. These studies demonstrate that four polypeptides whose phosphorylation is stimulated by the tumor promoter 12-O-tetradecanoyl phorbol 13-acetate in myotubes, are spontaneously phosphorylated in control myoblasts. Phosphoinositide turnover under basal conditions in [3H]inositol-labeled cells is faster in myoblasts than in myotubes, a finding that may in part explain the different distribution of PKc observed during the course of myogenic differentiation.

Signal transduction in the Sertoli cell: serum modulation of the response to FSH

Immature Sertoli cells of the testicular seminiferous tubule maintain the expression of their differentiated phenotype when cultured in unsupplemented medium. In preliminary experiments we observed that foetal bovine serum (FBS) stimulates polyphosphoinositides (PI) hydrolysis in Sertoli cells. We then evaluated the effect of serum on the function of the immature Sertoli cell in culture, in terms of cAMP and estrogen production. Treatment of Sertoli cells for 30 min with 1-10% FBS had no effect on basal cAMP accumulation but abolished the response to FSH. The serum concentration producing half-maximal inhibition of the FSH-dependent cAMP accumulation was 0.5-1%. Comparison of the FSH-dose-response in the absence or presence of serum showed a decreased maximal response when serum was present. Sertoli cells exposed to serum were also less responsive to the beta-adrenergic agonist isoproterenol, to cholera toxin, and to forskolin. The serum inhibition was rapidly reversed upon removal of serum or incubating the cells with the phosphodiesterase inhibitor MIX (methyl-isobutyl-xanthine). Similarly to what observed with cAMP, serum affected androgen aromatization stimulated by FSH, isoproterenol, cholera toxin, forskolin and dibutyryl cAMP. These data indicate that factors present in serum can act as modulators of the Sertoli cell function in vitro by rapidly and reversibly inhibiting the cAMP and steroidogenic response of the Sertoli cell to FSH.

Detection of cellular retinol-binding protein messenger RNA in the somatic cells of the rat seminiferous tubules

A cDNA clone coding for Cellular Retinol-Binding Protein (CRBP) was used as a probe to study the expression of the gene in the somatic cells of the seminiferous tubules (Sertoli and peritubular cells). In this paper we demonstrate that these cells are actively involved in the synthesis of the specific mRNA. In Sertoli cells the gene is modulated by the hormones effective in spermatogenesis, such as FSH and testosterone. Moreover, peritubular cells revealed an approximately two times higher concentration of CRBP steady-state mRNA levels when compared with Sertoli cells.

Gonadotropin-releasing hormone agonist activates protein kinase C in rat Leydig cells

Gonadotropin-releasing hormone (GnRH) has specific receptor sites in rat Leydig cells and has direct effects on their steroidogenesis. The purpose of the present study was to examine whether activation of the calcium- and phospholipid-dependent protein kinase C (PK-C) is involved in GnRH effects on rat Leydig cells, as has been shown in pituitary gonadotrophs. Testosterone production of Percoll-purified Leydig cells was similarly stimulated (about 50-100%) by a GnRH agonist (buserelin, maximum effect at concentration of 10(-9) mol/l and above) and a tumor promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA, maximum effect at 10(-8) mol/l), which is known to activate PK-C. In contrast, a GnRH antagonist (10(-5) mol/l) and an inactive phorbol ester, 4 alpha-phorbol-12,13-didecanoate (10(-6) mol/l), were without effect on testosterone. None of these substances had clear effects on cAMP production. The maximum steroidogenic effects of GnRH agonist and TPA were the same whether used separately or together, suggesting that they share a common mechanism of action. TPA translocated the cytosolic proportion of Leydig cell PK-C activity to a membrane-associated form almost instantaneously, within 0.5-1 min. A similar translocation, though less complete, was observed in the presence of buserelin in 1-4 min. Inclusion of a 100-fold excess of a potent GnRH antagonist completely prevented the translocation of PK-C. These results provide evidence that GnRH agonist activates PK-C also in the testis tissue, and this may be the mechanism whereby it affects Leydig cell endocrine function.

Inhibition by phorbol esters and other tumor promoters of the response of the Sertoli cell to FSH: evidence for dual site of action

Several tumor promoters exert their effects by activating a Ca2+-phospholipid-dependent protein kinase (protein kinase C). To study the role of this protein kinase in the regulation of Sertoli cell function, we have evaluated the effect of phorbol esters, mezerein, and teleocidin on the response of the Sertoli cell to FSH. Cells were treated for different time intervals with the tumor promoters, and cell response was measured by stimulating the cell with FSH. 12-O-Tetradecanoylphorbol 13-acetate (TPA) had no significant effect on basal cAMP production but markedly inhibited the cAMP response to FSH. Significant inhibition of cAMP accumulation was observed after 15 min treatment with 100 nM TPA, and maximal inhibition developed within 1 h. The decrease in cAMP accumulation was dependent on the dose of phorbol ester used, with an estimated ED50 of 10-20 nM TPA. In a manner similar to TPA, mezerein and teleocidin also inhibited the cAMP response of the Sertoli cell, while the phorbol ester 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), inactive as a tumor promoter and unable to stimulate protein kinase C activity, was devoid of effect. The promoters that inhibited cAMP response also inhibited the FSH-stimulated androgen aromatization. The dose of TPA producing half-maximal inhibition of estrogen accumulation was again 10-20 nM TPA, mezerein, and teleocidin inhibited estrogen accumulation whether FSH, forskolin or cholera toxin was used to stimulate the Sertoli cell. In contrast, only FSH-dependent cAMP accumulation was inhibited by the tumor promoters, while forskolin and cholera toxin stimulations were not affected. These data suggest that tumor promoters which activate protein kinase C act at two sites of the Sertoli cell response. They alter receptor-mediated signal transduction across the membrane and affect steroidogenesis at a site distal to cAMP accumulation.

Hormonal regulation of Sertoli cell function

The Sertoli cell is clearly influenced, directly and indirectly, by hormones. Among these are FSH, T, insulin and Vitamin A, but others may also be involved. Mechanisms are still not well understood. The biochemical effects of these hormones can be divided into quantitative and qualitative influences, with the former predominating. Specific cellular and secretory proteins and metabolites are affected, in many cases by more than one hormone. Often these same functions are influenced by other factors in the environment of the testis as well. Hormonal responsiveness of the Sertoli cell is determined in part by the maturational state of the cell. Some secreted products bind to specific cell types in the testis and epididymis and may influence the process of spermatogenesis. However, detailed mechanisms are not known at the present time. Understanding Sertoli cell function at the biochemical level and its control by hormones is clearly of key importance in understanding the control of the spermatogenic process.